You’ve got a rash. You quickly rule out the usual suspects: You haven’t been gardening or hiking or even picnicking, so it’s probably not a plant irritant such as poison ivy or wild parsnip; likewise, it’s probably not chiggers or ticks carrying Lyme disease; and you haven’t been swimming in a pond, which can harbor the parasite that causes swimmer’s itch.
Your rash may be coming from two hard-to-escape realities of summer: heat and sun.
The main difference between the two types of rashes is that a sun rash, like sunburn, occurs on sun-exposed skin. Heat rash occurs where the skin is covered.
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What causes it? Heat rash occurs when the sweat glands are blocked by tiny blisters – they resemble small beads of sweat – that form on the skin. Heat rash, called miliaria by doctors, can cause the skin to redden if the glands are blocked in a deeper layer of the skin.
The skin can feel irritated and itchy, giving rise to another common term for the syndrome – prickly heat.
The sequence – you sweat and then your sweat glands clog – is more likely to occur, says Herb Allen, a dermatologist and researcher at Drexel University in Philadelphia, “when you have too much clothing on – or too-tight clothes – or have been lying still on hospital bedsheets. There’s no air flow.”
Prevention and treatment: Fresh air, a breeze, air conditioning and fan-circulated air all help sweat evaporate and prevent heat rash. “Adequate ventilation is the key,” says Mary Sheu, medical director of the Johns Hopkins Dermatology & Cosmetic Center, near Baltimore. To prevent miliaria, wear lightweight clothing – and less of it, Allen says.
If you get it, a heavy-duty moisturizer, especially one containing lanolin, can provide relief, Allen says. Calamine lotion or cortisone cream can also help.
What causes it? Polymorphous light eruption (PMLE) is the most common culprit of sun rash, affecting about 10 to 20 percent of people in the U.S.
While every type of skin type can get sunburned, PMLE is an allergic reaction to the sun’s ultraviolet rays. Its form varies – it can produce an itchy rash or blisters or raised red blotches – and its severity varies widely. The reaction can emerge suddenly in one place, such as on arms exposed to the sun that first warm day in April.
“The condition tends to occur in the spring or early summer,” Sheu says. “It’s too much sun, too quickly.”
Prevention and treatment: The best protection is avoiding exposure to the sun altogether. Next best is wearing clothes that offer sun protection. Sunscreen products help, but they can’t entirely prevent PMLE. “Sunscreens are pretty good at blocking short UV rays, the UVB rays that cause sunburn,” Allen says. “But they’re not so good at blocking the long UV rays, the UVA rays. Even the best sunscreens block only 40 to 50 percent of UVA rays.”
Some sunscreens are better than others. Products labeled “broad-spectrum” are formulated to guard against both UVA and UVB rays. Those containing zinc or titanium oxide – these are known as physical blockers – are the best bet for people susceptible to PMLE reactions, Allen says.